VerbWhen a party commits to an occasion or arrangement, verbally or non-verbally, and fails to deliver on their initial commitment (usually pulling out of the commitment at the most inconvenient time).

The phrase emanates from Gandalf's consistently flaky actions in the Lord of the Rings series and in the Hobbit series.

In the Fellowship of the Ring, Gandalf leaves Frodo and Sam after their first meeting at Bag End, parting with a promise to rendezvous with them at Bree. Of course, Gandalf does not hold up his end of the bargain. Gandalf once again abandons the fellowship of the ring, this time more extravagantly, as he selfishly decides to pass into the afterlife after battling the Balrog in the Mines of Moria.

In the Two Towers, Gandalf abandons Aragon, Legolas, and others at Helm's Deep just before the battle begins.

In the Return of the King, Gandalf leaves middle earth as the sails for the Grey Havens, an Elvish paradise.

In the Hobbit, an Unexpected Journey, Gandalf leaves the company just before they are attacked by trolls.

In the Hobbit, the Desolation of Smaug, Gandalf bails on the company just before they enter Mirkwood. Subsequently, the company is captured by spiders and then elves.

The term used when someone driving assumes it is okay to indicate or directly change lanes before looking in their mirrors.
This confident action was named after Gandalf from the Lord of the Ring films. Assuming that he or she is the superior on the road and that others on it "shall not pass", hence the name, Gandalfing.

It is usually subtly mentioned by a passenger when they notice the driver committing this.
"Gandalf" - said under your breath when you notice them do it.
Alternatively, "are you gandalfing?" or,
"did you just Gandalf?"

When a guy is titty-fucking a girl, and when he cums, he cums on her chin and gives her a beard (like Gandalf).

A: "Hey man, guess what? I'm about to go Gandalfing"
B: "You're about to do what?!"
A: "Gandalfing. I'm about to go titty-fuck a girl, cum on her chin, and give her a beard... like Gandalf!"
B: "Dude, nice!"